[PATCH v5 1/1] modules: add modalias file to sysfs for modules.

Allen Webb
2022-12-01T21:17:04Z

USB devices support the authorized attribute which can be used by
user-space to implement trust-based systems for enabling USB devices. It
would be helpful when building these systems to be able to know in
advance which kernel drivers (or modules) are reachable from a
particular USB device.

This information is readily available for external modules in
modules.alias. However, builtin kernel modules are not covered. This
patch adds a sys-fs attribute to both builtin and loaded modules
exposing the matching rules in the modalias format for integration
with tools like USBGuard.

Note that as written CONFIG_MODULES must be enabled.

Signed-off-by: Allen Webb <allenwebb@google.com>
---
drivers/base/Makefile | 2 +-
drivers/base/base.h | 8 +
drivers/base/bus.c | 42 ++++++
drivers/base/mod_devicetable.c | 257 +++++++++++++++++++++++++++++++++
drivers/usb/core/driver.c | 2 +
include/linux/device/bus.h | 8 +
include/linux/module.h | 1 +
kernel/module/internal.h | 2 +
kernel/module/sysfs.c | 88 +++++++++++
kernel/params.c | 7 +
10 files changed, 416 insertions(+), 1 deletion(-)
create mode 100644 drivers/base/mod_devicetable.c

diff --git a/drivers/base/Makefile b/drivers/base/Makefile
index 83217d243c25b..924d46ae987f4 100644
--- a/drivers/base/Makefile
+++ b/drivers/base/Makefile
@@ -15,7 +15,7 @@ obj-y += firmware_loader/
obj-$(CONFIG_NUMA) += node.o
obj-$(CONFIG_MEMORY_HOTPLUG) += memory.o
ifeq ($(CONFIG_SYSFS),y)
-obj-$(CONFIG_MODULES) += module.o
+obj-$(CONFIG_MODULES) += mod_devicetable.o module.o
endif
obj-$(CONFIG_SYS_HYPERVISOR) += hypervisor.o
obj-$(CONFIG_REGMAP) += regmap/
diff --git a/drivers/base/base.h b/drivers/base/base.h
index b902d1ecc247f..fec56271104fa 100644
--- a/drivers/base/base.h
+++ b/drivers/base/base.h
@@ -173,6 +173,14 @@ static inline void module_add_driver(struct module *mod,
static inline void module_remove_driver(struct device_driver *drv) { }
#endif

+#if defined(CONFIG_SYSFS) && defined(CONFIG_MODULES)
+ssize_t usb_drv_to_modalias(struct device_driver *drv, char *buf,
+ size_t count);
+#else
+static inline ssize_t usb_drv_to_modalias(struct device_driver *drv, char *buf,
+ size_t count) { return -EINVAL; }
+#endif
+
#ifdef CONFIG_DEVTMPFS
extern int devtmpfs_init(void);
#else
diff --git a/drivers/base/bus.c b/drivers/base/bus.c
index 7ca47e5b3c1f4..4e0c5925545e5 100644
--- a/drivers/base/bus.c
+++ b/drivers/base/bus.c
@@ -178,6 +178,48 @@ static const struct kset_uevent_ops bus_uevent_ops = {

static struct kset *bus_kset;

+/**
+ * bus_for_each - bus iterator.
+ * @start: bus to start iterating from.
+ * @data: data for the callback.
+ * @fn: function to be called for each device.
+ *
+ * Iterate over list of buses, and call @fn for each,
+ * passing it @data. If @start is not NULL, we use that bus to
+ * begin iterating from.
+ *
+ * We check the return of @fn each time. If it returns anything
+ * other than 0, we break out and return that value.
+ *
+ * NOTE: The bus that returns a non-zero value is not retained
+ * in any way, nor is its refcount incremented. If the caller needs
+ * to retain this data, it should do so, and increment the reference
+ * count in the supplied callback.
+ */
+int bus_for_each(void *data, int (*fn)(struct bus_type *, void *))
+{
+ int error = 0;
+ struct bus_type *bus;
+ struct subsys_private *bus_prv;
+ struct kset *subsys;
+ struct kobject *k;
+
+ spin_lock(&bus_kset->list_lock);
+
+ list_for_each_entry(k, &bus_kset->list, entry) {
+ subsys = container_of(k, struct kset, kobj);
+ bus_prv = container_of(subsys, struct subsys_private, subsys);
+ bus = bus_prv->bus;
+ error = fn(bus, data);
+ if (error)
+ break;
+ }
+
+ spin_unlock(&bus_kset->list_lock);
+ return error;
+}
+EXPORT_SYMBOL_GPL(bus_for_each);
+
/* Manually detach a device from its associated driver. */
static ssize_t unbind_store(struct device_driver *drv, const char *buf,
size_t count)
diff --git a/drivers/base/mod_devicetable.c b/drivers/base/mod_devicetable.c
new file mode 100644
index 0000000000000..d7f198aad430f
--- /dev/null
+++ b/drivers/base/mod_devicetable.c
@@ -0,0 +1,257 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * mod_devicetable.c - helpers for displaying modaliases through sysfs.
+ *
+ * This borrows a lot from file2alias.c
+ */
+
+#include <linux/device/bus.h>
+#include <linux/device.h>
+#include <linux/usb.h>
+
+#include "base.h"
+#include "../usb/core/usb.h"
+
+/* Helper macro to add a modalias field to the string buffer associated with
+ * a match id.
+ *
+ * Note that:
+ * + len should be a ssize_t and is modified in the macro
+ * + sep should be a string literal and is concatenated as part of a format
+ * string
+ * + field is the struct field of the match id
+ */
+#define ADD(buf, count, len, sep, cond, field) \
+do { \
+ char *buf_ = buf; \
+ size_t count_ = count; \
+ if (cond) \
+ (len) += scnprintf(&buf_[len], \
+ count_ - (len), \
+ sizeof(field) == 1 ? (sep "%02X") : \
+ sizeof(field) == 2 ? (sep "%04X") : \
+ sizeof(field) == 4 ? (sep "%08X") : "", \
+ (field)); \
+ else \
+ (len) += scnprintf(&buf_[len], count_ - (len), (sep "*")); \
+} while (0)
+
+#ifdef CONFIG_USB
+/* USB related modaliases can be split because of device number matching, so
+ * this function handles individual modaliases for one segment of the range.
+ */
+static ssize_t usb_id_to_modalias(const struct usb_device_id *id,
+ unsigned int bcdDevice_initial,
+ int bcdDevice_initial_digits,
+ unsigned char range_lo,
+ unsigned char range_hi,
+ unsigned char max, const char *mod_name,
+ char *buf, size_t count)
+{
+ ssize_t len = 0;
+
+ ADD(buf, count, len, "alias usb:v",
+ id->match_flags & USB_DEVICE_ID_MATCH_VENDOR, id->idVendor);
+ ADD(buf, count, len, "p", id->match_flags & USB_DEVICE_ID_MATCH_PRODUCT,
+ id->idProduct);
+
+ len += scnprintf(&buf[len], count - len, "d");
+ if (bcdDevice_initial_digits)
+ len += scnprintf(&buf[len], count - len, "%0*X",
+ bcdDevice_initial_digits, bcdDevice_initial);
+ if (range_lo == range_hi) {
+ len += scnprintf(&buf[len], count - len, "%X", range_lo);
+ } else if (range_lo > 0 || range_hi < max) {
+ if (range_lo > 0x9 || range_hi < 0xA) {
+ len += scnprintf(&buf[len], count - len, "[%X-%X]",
+ range_lo, range_hi);
+ } else {
+ len += scnprintf(&buf[len], count - len,
+ range_lo < 0x9 ? "[%X-9" : "[%X",
+ range_lo);
+ len += scnprintf(&buf[len], count - len,
+ range_hi > 0xA ? "A-%X]" : "%X]",
+ range_hi);
+ }
+ }
+ if (bcdDevice_initial_digits < (sizeof(id->bcdDevice_lo) * 2 - 1))
+ len += scnprintf(&buf[len], count - len, "*");
+
+ ADD(buf, count, len, "dc",
+ id->match_flags & USB_DEVICE_ID_MATCH_DEV_CLASS, id->bDeviceClass);
+ ADD(buf, count, len, "dsc",
+ id->match_flags & USB_DEVICE_ID_MATCH_DEV_SUBCLASS,
+ id->bDeviceSubClass);
+ ADD(buf, count, len, "dp",
+ id->match_flags & USB_DEVICE_ID_MATCH_DEV_PROTOCOL,
+ id->bDeviceProtocol);
+ ADD(buf, count, len, "ic",
+ id->match_flags & USB_DEVICE_ID_MATCH_INT_CLASS,
+ id->bInterfaceClass);
+ ADD(buf, count, len, "isc",
+ id->match_flags & USB_DEVICE_ID_MATCH_INT_SUBCLASS,
+ id->bInterfaceSubClass);
+ ADD(buf, count, len, "ip",
+ id->match_flags & USB_DEVICE_ID_MATCH_INT_PROTOCOL,
+ id->bInterfaceProtocol);
+ ADD(buf, count, len, "in",
+ id->match_flags & USB_DEVICE_ID_MATCH_INT_NUMBER,
+ id->bInterfaceNumber);
+
+ len += scnprintf(&buf[len], count - len, " %s\n", mod_name);
+ return len;
+}
+
+/* Handles increment/decrement of BCD formatted integers */
+/* Returns the previous value, so it works like i++ or i-- */
+static unsigned int incbcd(unsigned int *bcd,
+ int inc,
+ unsigned char max,
+ size_t chars)
+{
+ unsigned int init = *bcd, i, j;
+ unsigned long long c, dec = 0, div;
+
+ /* If bcd is not in BCD format, just increment */
+ if (max > 0x9) {
+ *bcd += inc;
+ return init;
+ }
+
+ /* Convert BCD to Decimal */
+ for (i = 0 ; i < chars ; i++) {
+ c = (*bcd >> (i << 2)) & 0xf;
+ c = c > 9 ? 9 : c; /* force to bcd just in case */
+ for (j = 0 ; j < i ; j++)
+ c = c * 10;
+ dec += c;
+ }
+
+ /* Do our increment/decrement */
+ dec += inc;
+ *bcd = 0;
+
+ /* Convert back to BCD */
+ for (i = 0 ; i < chars ; i++) {
+ for (c = 1, j = 0 ; j < i ; j++)
+ c = c * 10;
+ div = dec;
+ (void)do_div(div, c); /* div = div / c */
+ c = do_div(div, 10); /* c = div % 10; div = div / 10 */
+ *bcd += c << (i << 2);
+ }
+ return init;
+}
+
+/* Print the modaliases for the specified struct usb_device_id. */
+static ssize_t usb_id_to_modalias_multi(const struct usb_device_id *id,
+ const char *mod_name, char *buf,
+ size_t count)
+{
+ ssize_t len = 0;
+ unsigned int devlo, devhi;
+ unsigned char chi, clo, max;
+ int ndigits;
+
+ devlo = id->match_flags & USB_DEVICE_ID_MATCH_DEV_LO ?
+ id->bcdDevice_lo : 0x0U;
+ devhi = id->match_flags & USB_DEVICE_ID_MATCH_DEV_HI ?
+ id->bcdDevice_hi : ~0x0U;
+
+ /* Figure out if this entry is in bcd or hex format */
+ max = 0x9; /* Default to decimal format */
+ for (ndigits = 0 ; ndigits < sizeof(id->bcdDevice_lo) * 2 ; ndigits++) {
+ clo = (devlo >> (ndigits << 2)) & 0xf;
+ chi = ((devhi > 0x9999 ? 0x9999 : devhi) >>
+ (ndigits << 2)) & 0xf;
+ if (clo > max || chi > max) {
+ max = 0xf;
+ break;
+ }
+ }
+
+ /*
+ * Some modules (visor) have empty slots as placeholder for
+ * run-time specification that results in catch-all alias
+ */
+ if (!(id->idVendor || id->idProduct || id->bDeviceClass ||
+ id->bInterfaceClass))
+ return len;
+
+ /* Convert numeric bcdDevice range into fnmatch-able pattern(s) */
+ for (ndigits = sizeof(id->bcdDevice_lo) * 2 - 1; devlo <= devhi;
+ ndigits--) {
+ clo = devlo & 0xf;
+ chi = devhi & 0xf;
+ /* If we are in bcd mode, truncate if necessary */
+ if (chi > max)
+ chi = max;
+ devlo >>= 4;
+ devhi >>= 4;
+
+ if (devlo == devhi || !ndigits) {
+ len += usb_id_to_modalias(id, devlo, ndigits, clo, chi,
+ max, mod_name, buf + len,
+ count - len);
+ break;
+ }
+
+ if (clo > 0x0)
+ len += usb_id_to_modalias(id,
+ incbcd(&devlo, 1, max,
+ sizeof(id->bcdDevice_lo) * 2),
+ ndigits, clo, max, max, mod_name, buf + len,
+ count - len);
+
+ if (chi < max)
+ len += usb_id_to_modalias(id,
+ incbcd(&devhi, -1, max,
+ sizeof(id->bcdDevice_lo) * 2),
+ ndigits, 0x0, chi, max, mod_name, buf + len,
+ count - len);
+ }
+ return len;
+}
+
+/* Print the modaliases for the given driver assumed to be an usb_driver or
+ * usb_device_driver.
+ *
+ * "alias" is prepended and the module name is appended to each modalias to
+ * match the format in modules.aliases.
+ *
+ * The modaliases will be written out to @buf with @count being the maximum
+ * bytes to write. The return value is a negative errno on error or the number
+ * of bytes written to @buf on success.
+ */
+ssize_t usb_drv_to_modalias(struct device_driver *drv, char *buf,
+ size_t count)
+{
+ ssize_t len = 0;
+ const struct usb_device_id *id;
+ const char *mod_name;
+
+ if (drv->bus != &usb_bus_type)
+ return -EINVAL;
+
+ if (drv->owner)
+ mod_name = drv->owner->name;
+ else
+ mod_name = drv->mod_name;
+
+ if (is_usb_device_driver(drv))
+ id = to_usb_device_driver(drv)->id_table;
+ else
+ id = to_usb_driver(drv)->id_table;
+ if (!id)
+ return len;
+
+ for (; id->match_flags; id++) {
+ len += usb_id_to_modalias_multi(id, mod_name, buf + len,
+ count - len);
+ }
+ return len;
+}
+#else
+inline ssize_t usb_drv_to_modalias(struct device_driver *drv, char *buf,
+ size_t count){ return 0; }
+#endif
diff --git a/drivers/usb/core/driver.c b/drivers/usb/core/driver.c
index 7e7e119c253fb..fdbc197b64c9c 100644
--- a/drivers/usb/core/driver.c
+++ b/drivers/usb/core/driver.c
@@ -32,6 +32,7 @@
#include <linux/usb/quirks.h>
#include <linux/usb/hcd.h>

+#include "../../base/base.h"
#include "usb.h"

@@ -2030,4 +2031,5 @@ struct bus_type usb_bus_type = {
.match = usb_device_match,
.uevent = usb_uevent,
.need_parent_lock = true,
+ .drv_to_modalias = usb_drv_to_modalias,
};
diff --git a/include/linux/device/bus.h b/include/linux/device/bus.h
index d8b29ccd07e56..cce0bedec63d9 100644
--- a/include/linux/device/bus.h
+++ b/include/linux/device/bus.h
@@ -61,6 +61,10 @@ struct fwnode_handle;
* this bus.
* @dma_cleanup: Called to cleanup DMA configuration on a device on
* this bus.
+ * @drv_to_modalias: Called to convert the matching IDs in a
+ * struct device_driver to their corresponding modaliases.
+ * Note that the struct device_driver is expected to belong
+ * to this bus.
* @pm: Power management operations of this bus, callback the specific
* device driver's pm-ops.
* @iommu_ops: IOMMU specific operations for this bus, used to attach IOMMU
@@ -107,6 +111,9 @@ struct bus_type {
int (*dma_configure)(struct device *dev);
void (*dma_cleanup)(struct device *dev);

+ ssize_t (*drv_to_modalias)(struct device_driver *drv, char *buf,
+ size_t count);
+
const struct dev_pm_ops *pm;

const struct iommu_ops *iommu_ops;
@@ -161,6 +168,7 @@ void subsys_dev_iter_init(struct subsys_dev_iter *iter,
struct device *subsys_dev_iter_next(struct subsys_dev_iter *iter);
void subsys_dev_iter_exit(struct subsys_dev_iter *iter);

+int bus_for_each(void *data, int (*fn)(struct bus_type *, void *));
int bus_for_each_dev(struct bus_type *bus, struct device *start, void *data,
int (*fn)(struct device *dev, void *data));
struct device *bus_find_device(struct bus_type *bus, struct device *start,
diff --git a/include/linux/module.h b/include/linux/module.h
index ec61fb53979a9..0bfa859a21566 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -47,6 +47,7 @@ struct module_kobject {
struct kobject *drivers_dir;
struct module_param_attrs *mp;
struct completion *kobj_completion;
+ struct bin_attribute modalias_attr;
} __randomize_layout;

struct module_attribute {
diff --git a/kernel/module/internal.h b/kernel/module/internal.h
index 2e2bf236f5582..8d7ae37584868 100644
--- a/kernel/module/internal.h
+++ b/kernel/module/internal.h
@@ -259,11 +259,13 @@ static inline void add_kallsyms(struct module *mod, const struct load_info *info
#endif /* CONFIG_KALLSYMS */

#ifdef CONFIG_SYSFS
+void add_modalias_attr(struct module_kobject *mk);
int mod_sysfs_setup(struct module *mod, const struct load_info *info,
struct kernel_param *kparam, unsigned int num_params);
void mod_sysfs_teardown(struct module *mod);
void init_param_lock(struct module *mod);
#else /* !CONFIG_SYSFS */
+static inline void add_modalias_attr(struct module_kobject *mk) {}
static inline int mod_sysfs_setup(struct module *mod,
const struct load_info *info,
struct kernel_param *kparam,
diff --git a/kernel/module/sysfs.c b/kernel/module/sysfs.c
index ce68f821dcd12..651c677c4ab96 100644
--- a/kernel/module/sysfs.c
+++ b/kernel/module/sysfs.c
@@ -5,6 +5,8 @@
* Copyright (C) 2008 Rusty Russell
*/

+#include <linux/device/bus.h>
+#include <linux/device/driver.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/fs.h>
@@ -240,6 +242,90 @@ static inline void add_notes_attrs(struct module *mod, const struct load_info *i
static inline void remove_notes_attrs(struct module *mod) { }
#endif /* CONFIG_KALLSYMS */

+/* Track of the buffer and module identity in callbacks when walking the list of
+ * drivers for each bus.
+ */
+struct modalias_bus_print_state {
+ struct module_kobject *mk;
+ char *buf;
+ size_t count;
+ ssize_t len;
+};
+
+static int print_modalias_for_drv(struct device_driver *drv, void *p)
+{
+ struct modalias_bus_print_state *s = p;
+ struct module_kobject *mk = s->mk;
+ ssize_t len;
+ /* Skip drivers that do not match this module. */
+ if (mk->mod) {
+ if (mk->mod != drv->owner)
+ return 0;
+ } else if (!mk->kobj.name || !drv->mod_name ||
+ strcmp(mk->kobj.name, drv->mod_name))
+ return 0;
+
+ if (drv->bus && drv->bus->drv_to_modalias) {
+ len = drv->bus->drv_to_modalias(drv, s->buf + s->len,
+ s->count - s->len);
+ if (len < 0)
+ return len;
+ s->len += len;
+ }
+ return 0;
+}
+
+static int print_modalias_for_bus(struct bus_type *type, void *p)
+{
+ return bus_for_each_drv(type, NULL, p, print_modalias_for_drv);
+}
+
+static ssize_t module_modalias_read(struct file *filp, struct kobject *kobj,
+ struct bin_attribute *bin_attr,
+ char *buf, loff_t pos, size_t count)
+{
+ struct module_kobject *mk = container_of(kobj, struct module_kobject,
+ kobj);
+ struct modalias_bus_print_state state = {mk, buf, count, 0};
+ int error = 0;
+
+ if (pos != 0)
+ return -EINVAL;
+
+ error = bus_for_each(&state, print_modalias_for_bus);
+ if (error)
+ return error;
+
+ /*
+ * The caller checked the pos and count against our size.
+ */
+ return state.len;
+}
+
+/* Used in kernel/params.c for builtin modules.
+ *
+ * `struct module_kobject` is used instead of `struct module` because for
+ * builtin modules, the `struct module` is not available when this is called.
+ */
+void add_modalias_attr(struct module_kobject *mk)
+{
+ sysfs_bin_attr_init(&mk->modalias_attr);
+ mk->modalias_attr.attr.name = "modalias";
+ mk->modalias_attr.attr.mode = 0444;
+ mk->modalias_attr.read = module_modalias_read;
+ if (sysfs_create_bin_file(&mk->kobj, &mk->modalias_attr)) {
+ /* We shouldn't ignore the return type, but there is nothing to
+ * do.
+ */
+ return;
+ }
+}
+
+static void remove_modalias_attr(struct module_kobject *mk)
+{
+ sysfs_remove_bin_file(&mk->kobj, &mk->modalias_attr);
+}
+
static void del_usage_links(struct module *mod)
{
#ifdef CONFIG_MODULE_UNLOAD
@@ -398,6 +484,7 @@ int mod_sysfs_setup(struct module *mod,

add_sect_attrs(mod, info);
add_notes_attrs(mod, info);
+ add_modalias_attr(&mod->mkobj);

return 0;

@@ -415,6 +502,7 @@ int mod_sysfs_setup(struct module *mod,

static void mod_sysfs_fini(struct module *mod)
{
+ remove_modalias_attr(&mod->mkobj);
remove_notes_attrs(mod);
remove_sect_attrs(mod);
mod_kobject_put(mod);
diff --git a/kernel/params.c b/kernel/params.c
index 5b92310425c50..b7fd5313a3118 100644
--- a/kernel/params.c
+++ b/kernel/params.c
@@ -14,6 +14,12 @@
#include <linux/ctype.h>
#include <linux/security.h>

+#ifdef CONFIG_MODULES
+#include "module/internal.h"
+#else
+static inline void add_modalias_attr(struct module_kobject *mk) {}
+#endif /* !CONFIG_MODULES */
+
#ifdef CONFIG_SYSFS
/* Protects all built-in parameters, modules use their own param_lock */
static DEFINE_MUTEX(param_lock);
@@ -815,6 +821,7 @@ static void __init kernel_add_sysfs_param(const char *name,
BUG_ON(err);
kobject_uevent(&mk->kobj, KOBJ_ADD);
kobject_put(&mk->kobj);
+ add_modalias_attr(mk);
}

/*
--
2.37.3